Recently, electrophotographic technology has been widely applied to a variety of printers, as well as the field of copiers, due to its immediacy and formation of high-quality images. Electrophotographic photoreceptors (hereinafter, referred to as “photoreceptor” simply) lie in the core technology of electrophotography, and organic photoreceptors using organic photoconductive materials have been developed, since they have advantages such as non-pollution and ease in production in comparison with inorganic photoconductive materials.
In general, an organic photoreceptor is composed of an electroconductive support and a photosensitive layer disposed thereon. Photoreceptors are classified into, for example, a so-called single-layer photoreceptor having a single photosensitive layer (single photosensitive layer) containing a binder resin dissolving or dispersing a photoconductive material therein; and a so-called multilayered photoreceptor composed of a plurality of laminated layers (laminated photosensitive layer) including a charge-generating layer containing a charge-generating material and a charge-transporting layer containing a charge-transporting material.
In the organic photoreceptor, changes in use environment of the photoreceptor or changes in electric characteristics during repeated use may cause various defects in an image formed with the photoreceptor. As a technology to solve such problems, a reliable method for forming a good image is to use an undercoat layer that contains a binder resin and titanium oxide particles and is disposed between an electroconductive substrate and a photosensitive layer (for example, refer to Patent Document 1).
The layer of the organic photoreceptor is generally formed by applying and drying a coating liquid prepared by dissolving or dispersing a material in a solvent, because of its high productivity. In such a case, since the titanium oxide particles and the binder resin are incompatible with each other in the undercoat layer, the coating liquid for forming the undercoat layer is provided in the form of a dispersion of titanium oxide particles.
Such a coating liquid has been generally produced by wet-dispersing titanium oxide particles in an organic solvent using a known mechanical pulverizer, such as a ball mill, a sand grind mill, a planetary mill, or a roll mill, by spending a long period of time (for example, refer to Patent Document 1). Furthermore, it is disclosed that when titanium oxide particles are dispersed in a coating liquid for forming an undercoat layer using a dispersion medium, an electrophotographic photoreceptor that exhibits excellent characteristics in repeated charging-exposure cycles even under conditions of low temperature and low humidity can be provided using titania or zirconia as the dispersion medium (for example, refer to Patent Document 2).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. Hei 11-202519
[Patent Document 2] Japanese Unexamined Patent Application Publication No. Hei 6-273962